1. Field of the Invention
The invention relates to a method for treating a paper or cardboard web, in which method a coat is applied to a web and the web is then glazed. The invention further relates to a device for treating a paper or cardboard web with a coating device and a calender.
2. Discussion of Background Information
Paper and cardboard webs are often coated, i.e., provided with a coating. The coat, also referred to as “coating composition,” is used to give the web a better surface property. In particular the gloss and/or smoothness at the coated surface of the web, i.e., the surface provided with a coating, can be improved with a coat.
After the application of the coat, the surface of the web is often still relatively coarse. Therefore, the web must be glazed to achieve again certain smoothness values and to fix the coat to the web. To this end, relatively high compressive stresses and often also relatively high temperatures are required. This renders glazing a relatively high-expenditure process.
With non-coated papers, the compressive stresses in the calender nip and thus the strains on the web can be reduced by moistening the web before the entry into a nip. To this end, the web is frequently guided through a supercalender if high gloss and smoothness values are desired. With more simply designed webs, a machine calender or soft calender is also sufficient, in which the web is guided through merely one to four hard or soft nips, which are embodied in roll stacks featuring two or three rolls each.
To date, any attempts to improve the glazing result of coated webs through the use of steam have failed. As soon as steam is applied to the coat, the coat becomes sticky and in the calender adheres to the roll, against which the coated side of the web lies. Typically, this is a hard, heated roll. Within the shortest of times this roll is then soiled to such an extent that the glazing or the smoothing process must be interrupted. Therefore, such a procedure has proven to be unsuitable in practice.
Consequently, with coated papers, it is still necessary to operate without a moistening device, but with relatively high line loads and the ensuing relatively high compressive stresses; with higher-quality papers, even with a supercalender which requires a correspondingly high number of rolls. This has so far been the only way to achieve the desired high gloss and smoothness values. But the high line loads have the disadvantage that the web is greatly condensed and that the volume and stiffness of the papers decrease considerably.